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deposition of glacio-fluvial material. In inter-Glacial time, the process of
rejuvenation and piracy, begun prior to glaciation, was resumed in some
areas. In other places, disruption of the pre-Glacial drainage pattern by
glacial erosion and deposition was followed by deposition of considerable
alluvial or lacustrine material. During the periods of ice occupation,
channels were cut in drift and in bedrock along the sides of valley glaciers;
many of these, floored with boulders derived from rock formations exposed
farther upstream, are now preserved high and dry on the mountainside,
associated with boulder paths and various kame and pro-glacial lake
deposits. The natural outlets of several valleys became blocked with ice,
and in some cases with drift, forcing the streams to cut new channels
through the valley wall or across mountain spurs.

Subsequent to the retreat of the last large,valley glaciers, a cycle of
canyon cutting commenced. This cycle is still in its early stages, and has
resulted in long gorges at the mouths of several low-lying tributary valleys.
The amount of notching of the lips of higher hanging valleys and cirque
basins has been negligible.

DRAINAGE CHANGES

The drainage pattern of a large part of the map-area has been
profoundly altered since pre-Glacial times. The suggested histories of two
river courses that have undergone relatively large-scale rearrangement are
presented here as illustrations of the type of drainage changes that produced
the present stream pattern. The interpretations are tentative, and may
require modification as more evidence becomes available.

SWANNELL RIVER

The present Swannell River has four distinct parts, separated by three,
roughly right-angle bends. The upper reaches lie in a broad, relatively
high-level valley that separates Ingenika and Lay Ranges and, now
occupied by the headwaters of Wrede Creek, continues west of the map-
area. This valley probably drained originally to Mesilinka Valley near
the present Blackpine Lake. Some time before the area was occupied by
glaciers, this relatively high-level valley was beheaded by a tributary of
what is now Wrede Creek; robbed of much of its water supply, the
remainder was lowered by stream erosion more slowly than the surround-
ing valleys, and its drainage was ultimately captured east of Mount Lay
by a tributary of Ingenika River, eroding southward along the line of
a fault zone in what is now a large through valley. Thus the first right-
angle bend was established.

Prolonged and perhaps repeated glaciation enlarged and deepened the
through valley east of Mount Lay, and similarly deepened Mesilinka Valley
to the south, leaving the Swannell-Mesilinka pass in about its present
position. A trunk glacier occupied Ingenika Valley; streams flowing along
its side cut a channel across the end of Chase Mountain. Retreat of the
glacier in Swannell Valley apparently began while Ingenika Valley was
still largely filled with ice. This ice blocked the outlet of Swannell Valley,
and diverted meltwater from the Swannell Valley glacier, through the

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